The typical motors used for gauges in vehicles to indicate engine revolutions per minute (RPM) are of the cross-coil type and are sometimes referred to sine/cosine, air core, or D'Arsonval meters/motors. In addition, some gauges use a stepper motor to provide the drive force to position a pointer to indicate a reading to the vehicle operator.
In many of the previous systems, the pointer has a limited rotation angle due to the type of meter or motor used. Another limitation of previous systems is that the pointer often has a nonlinear position movement in relation to the input voltage. In some previous systems, lookup tables have been used to attempt to correct for this non-linearity while, in other systems, microprocessors have been employed in an attempt to calculate offset correction values.
Even with the use of correction tables and microprocessors, these previous systems can not indicate an accurate value for RPM or any measurement better than plus or minus one and one-half percent of the correct value across the full tachometer RPM range. This equates to over 180 RPM error at a 12,000-RPM full scale reading. Also, since this error is not constant and is much larger at the extreme ends of the angular rotation, the lowest and highest RPM values typically have the most display error.
Still another shortcoming of the air core meter is the acceleration capability of the air core meter to rapidly swing the pointer so that the RPM pointer does not lag behind the actual engine RPM. Many meter movements have limited position acceleration rates to avoid pointer jitter at low engine RPM when the input RPM data is at a low repetition rate.
In some previous systems a constant light source was used to illuminate the meter pointer. For example, in some previous systems, several light sources were embedded in a translucent material to conduct light to the center pointer shaft, which caused light to reflect along the pointer shaft. In other previous systems, rotating springs were connected to the pointer shaft to apply a power input to the pointer, which contained an embedded light source. Still other systems used a translucent material to mount both the light source and the drive circuit, thereby illuminating the faceplate and pointer. In yet other systems, multiple LEDs were used to backlight a tachometer or other gauge, which also illuminated the pointer in the center of the illuminated display.
However, in all of these previous systems, no control of the light source was provided. Moreover, the systems often had complex arrangements for providing and maintaining the light source and these arrangements were difficult and/or costly to construct and maintain.
Furthermore, in many previous systems, external switches were used for functions such as peak RPM reset and RPM switch function programming. However, these approaches led to complex devices requiring a cable from the tachometer to a switch module, mounts for the switch module, and frequent noise interference coupled into the external module or cable connecting the tachometer and switch module. Moreover, these mechanical switches were often subject to vibration, electrical and mechanical wear, and/or damage to the interconnecting cable or possible interference by the cable and other system components.